Process of drying polychloroprene sheets



C. J. DEFIEL PROCESS OF DRYING POLYCHLOROPRENE SHEETS I June 7, 1966 2 Sheets-Sheet 1 Filed July 10, 1962 INVENTOR U/AIPA 7v/v 52W; wit

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United States Patent "ice 3,254,422 PROCESS OF DRYING POLYCHLOROPRENE SHEETS Charlton John Defiel, Anchorage, Ky., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware Filed July 10, 1962, Ser. No. 208,789 4 Claims. (Cl. 34-18) This invention relates to a new process for rapidly drying a plastic polymer sheet which is obtained from an aqueous dispersion of chloroprene polymer.

Methods available for drying chloroprene polymer obtained in sheet form from a latex include heating in drying chambers or exposing to currents of hot air. These methods require long exposure times and hence excessive drying equipment unless high temperatures are used, in which instance the polymers may be undesirably altered. Infrared radiation used by itself to effect drying temperatures below 95 C. or in combination with hot air at similar temperatures has been employed, but again undesirably long exposure times are required. At higher temperatures caused by more intense infrared radiation, or longer exposure thereto, both with or without hot air, alteration of the polymer occurs. A quicker method than taught by the art of drying chloroprene polymer sheet without alteration thereof is desired.

The object of this invention is to provide a process for rapidly drying a chloroprene polymer sheet which contains a large amount of water such as is found after preliminary Water removing steps in a chloroprene polymer sheet which is obtained by coagulating into web form the polymer contained in a latex. An additional object of this invention is to provide a process which does not alter the polymer during rapid drying. A still further object of this invention is to provide a novel combination of infrared radiation and cool-to-warm air to achieve the improvements hereinbefore described. Other objects will appear hereinafter.

These and other objects are accomplished by the process which comprises exposing a wet chloroprene polymer sheet, obtained by coagulating a chloroprene 'polymer latex, simultaneously to infrared radiation and to a current of air at a maximum temperature of about 75 C., the amounts of radiation and of air being regulated so that the temperature of the sheet being dried is between 100 and 160C.

The present invention is best carried out continuously by passing a supported, continuous polymer sheet to be dried past a series of sources of infrared radiation which are associated with multiple jets of cool air, the

latter for controlling the sheet temperature. Air is preferred for convenience and economy, but any gas which is inert to the sheet at temperatures below 160 C. is applicable. The infrared radiation employed should have its maximum emission at wavelengths between 2 and 4 microns. The sheet before drying is preferably between 0.02 and 0.08 inch thick and contains between 20-50% water, based on the weight of the dry polymer. Thinner and thicker sheets of greater or less moisture content can also be effectively dried by the process of this invention. 4

The process of the present invention is particularly 3,254,422 Patented June 7, 1966 useful for drying the thick, fairly non-porous sheets formed by chemical coagulation of a chloroprene polymer latex between rotating rolls. The preferred coagulating agent is an aqueous solution containing 0.10 to 0.50% by weight of calcium chloride, 0.05 to 0.15% by weight of hydroxyethyl cellulose, and enough hydrochloric acid to maintain a pH below 6.5 during coagulation.

The polymers to which the present invention is particularly applicable are those plastic or plasticizable polymers of chloroprene made in the presence of any of the chain terminating or other modifying agents known in the art, especially aliphatic mercaptans, iodine, sulfur, and the dialkyl xanthogen disulfides. The chloroprene polymers processed include the homopolymer and copolymers thereof with less than 50% of the total weight of the copolymer of other copolymerizable mono mers such as styrene, acrylonitrile, and 2,3-dichlorobutadiene-1,3.

To form the chloroprene polymer sheet, a latex containing usually from 25 to 50% by weight of polymer solids is simultaneously contacted with coagulating solution and passed through the nip formed between two cylindrical members spaced apart and both'rotating in the direction of flow of the latex and coagulating solution. The roll surfaces are preferably wet with coagulating solution and then the latex is applied to the nip between said members at a rate to maintain an excess of coagulum in the trough formed above the nip by the cylindrical members; The'coagulum after passing between said members is recovered as a uniform, continuous sheet. The sheet is alternately squeezed between rolls to remove salt solution and contacted with water, the washing process ending with a squeezing operation.

The sheet thus made is then dried according to the process of the present invention for which the following drawings depict representative equipment. 7

FIG. 1 shows a schematic side view of a drier for carrying out the process of the present invention.

FIG. 2 is a cross-sectional view taken along line AA of FIG. 1.

In FIG. 1, the sheet 10 to be dried is received upon an endless conveyor 11 on which it is supported or held in place by horizontal crossbars 12 or girts, shown for only parts of the conveyor, and is passed through drying section I. This section consists of two verticalbanks of infrared heaters 13. These heaters are commercially available as rectangular ceramic grids which are heated by gas passing through the grids and burning on the outer surfaces thereof. Heaters 13 are arranged two abreast as shown or in a staggered relationship. Said heaters are further arranged in vertical banks with the emission surfaces facing each other. Heat-reflecting aluminum or stainless steel partitions 14 separate the opposing heaters of each pair of banks. Conveyor 11 carries sheet 10 upward past the first bank of heaters of section I and then downward past the second bank and so on through succeeding sections II and HI which are the same as section I. Jets of air through horizontally spaced perforations ,in horizontal T-tubes 15 (see FIG. 2) are directed against the sheet as it moves past the heaters 13.

at the required distance from the heaters. FIG. 2, which shows a horizontal cross section at A-A through the drier of FIG. 1, shows how the air jets, depicted as arrows, are produced through the perforations in T-tubes 15. Manifold pipes 16 supply air to said T-tubes.

The amount of the infrared radiation supplied to the sheet is conveniently varied by changing the number of heaters. The intensity is varied by regulating the source of heat therefor and the distance from the sheet. The speed of the conveyor, the volume of air through the jets, and the distances of the air jets from the sheet are also variable and affect the rate of drying. The number of sections I-III necessary will depend upon the choice of these variables. The whole assembly is partly en- 'closed, with ample space provided for the exit of the combustion gases and of the warm air containing the moisture from the sheet.

The dry sheet thus produced may be gathered and compressed into a rope which is then cut into short sections and packaged, all as described in US. Patent 2,349,- 829.

Alternatively, the conveyor may be arranged so that the sheet to be dried travels in a horizontal or inclined path past infrared heaters, with their radiating surfaces parallel to the sheet. The infrared radiators may be electrically heated. Infrared radiation along with cooling air may be applied simultaneously to both sides of the sheet if horizontal conveying is used.

The rapid drying obtained in the process of the present invention results from the use of infrared radiation intense enough, if acting alone, to heat the sheet of chloroprene polymer above 100-160 C., combined with the cooling action upon the sheet of current of cool air so that the sheet temperature is maintained between 100 and 160 C. If the radiation is only enough to maintain this temperature range, then the rate of drying is impractically low. If, on the other hand, this range is exceeded in an attempt to get faster drying, the sheet darkens and becomes unstable, changing rapidly in plasticity on storage. Thus the amount of radiation received by the sheet and the amount of cooling air supplied must be balanced so as to maintain the film between 100 and 160 C. The plasticity of chloroprene polymer sheets dried by the present invention does not change significantly during storage.

While the present invention has been explained in terms of a chloroprene polymer sheet which is prepared in a preferred manner hereinbefore described, the practice of the invention is not so limited. Sheets of chloroprene polymer prepared by other methods which avoid coagulation of a latex into large, impervious masses can be dried by the present novel process. One such process involves freezing the polymer latex in thin layers, thus giving, after melting of the ice crystals, a porous coagulum which is easily washed free of remaining electrolytes. See US. 2,187,146. This process has the disadvantage of rather high operating costs because of the refrigeration and additional chemicals required.

Another process is to extrude the latex in a fine stream through a narrow slit formed by stationary sidewalls into a coagulating bath, thus forming a continuous thin sheet of coagulum which is then washed. This sheet, as well as the thin sheet made by freezing in the first method described above, are-advantageously dried by the infrared method of the present invention.

Representative examples of the present invention are as follows. Parts and percents are by weight unless otherwise indicated.

Example 1 A polychloroprene latex is made by polymerizing at 40 C. chloroprene in the presence of 0.25% by weight dodecyl mercaptan in an alkaline aqueous emulsion in which the dispersing agent is the sodium salt of disproportionated rosin. The latex is coagulated by passage in the presence of a coagulant water solution, containing 0.20% by Weight calcium chloride, sufficient hydrochloric acid to maintain a pH of lms than 6.5 during coagulation, and 0.05% by weight hydroxyethyl cellulose, through the space between two rolls both rotating in the direction of flow of the latex and coagulant. The resulting continuous sheet of coagulum is washed to remove electrolytes by alternate squeezing and spraying with water during relaxation. After a final squeezing, the sheet is 0.030-inch thick and 12 inches wide and contains 30% water and is ready for drying according to the present invention.

The drying equipment is of the general construction of the drier described in FIG. 1 and has a conveyor moving at 9 feet per minute. The sheet is in contact with the conveyor for 35 feet. The infrared radiation is provided by 16 heaters, each with a radiating surface approximately 6 inches square, operating with the emitting surface at 1600 F. with the peak radiation at a wave length of 2.7 microns, with a total radiation in the infrared region of about 6500 B.t.u. per hour. The air, introduced at a speed of about 4 feet per second and 21 to 32 C. (70 to F.), amounts to about 40 cubic feet per minute per burner. The burners are arranged 6 inches from the sheet on the conveyor. Ten of them are arranged two abreast and 6 in staggered positions, along the path of the conveyor. Thus the sheet is exposed to the radiation for a distance of 5.5 feet and a total time of 36.7 seconds. The temperature of the sheet as it passes through the drier is between and C. (212 and 320 F.). The sheet on leaving the drier is light in color and contains only 0.5% moisture. The material is plastic and does not change significantly in plasticity during storage and is readily formed into a solid rope and cut into pieces, suitable for packaging, storage, and conversion to elastomer stocks by conventional milling and compounding operations.

Example 2 The latex used in Example 1 is coagulated by the process described there to give a sheet 12 inches wide and 0.060 inch thick and containing 36% water. The drier is modified to provide for the thicker and wetter film by reducing the conveyor speed to 4.5 feet per minute and increasing the heaters to 26. Of these, 16 are arranged two abreast and 10 staggered, to give an exposure path of 9 feet and an exposure time of 2 minutes. Each heater has its accompanying jets of cooling air at 40 cubic feet per minute. The resulting dried material is like that of Example 1, containing 0.6% moisture.

In the foregoing examples, polymer sheets derived from latices containing copolymers of polychloroprene with other copolymerizable monomers as previously described will give similar results, and accordingly, this invention is not limited to drying the homopolymer in web form.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims.

What is claimed is:

1. A process for drying wet chloroprene polymer while supported in the form of a sheet on a grid surface which comprises exposing said sheet to a source of sufficient infrared radiation, which in the absence of air cooling would heat said sheet to a temperature greater than 160 C., concurrently directing a current of air at a temperature less than 75 C. at said sheet and its supporting grid surface to maintain the temperature of said sheet between 100 and 160 C. and to maintain said sheet in contact with said supporting grid surface.

2. Process of claim 1 wherein the infrared radiation has a maximum emission in the range of 2-4 microns.

3. The process of claim 2 wherein the said current of air is maintained at an initial temperature of about 21 to, 32 C.

5 4. Process of claim 3 wherein the said sheet has a uniform thickness between 0.02 and 0.08 inch and contains from 2050% Water based on the weight of dry polymers.

References Cited by the Examiner UNITED STATES PATENTS 12/1937 Wellmar 34-18 6 2,751,365 6/1956 Scott 260-'92.3 3,054,192 9/1962 Rosahl 34-4 3,151,950 10/ 1964 Newman et a1 34-4 5 WILLIAM F. ODEA, Primary Examiner.

NORMAN YUDKOFF, Examiner.

W. C. EVERETT, JOHN J. CAMBY,

Assistant Examiners. 

1. A PROCESS FOR DRYING WET CHLOROPRENE POLYMER WHILE SUPPORTED IN THE FORM OF A SHEET ON A GRID SURFACE WHICH COMPRISES EXPOSING SAID SHEET TO A SOURCE OF SUFFICIENT INFRARED RADIATION, WHICH IN THE ABSENCE OF AIR COOLING WOULD HEAT SAID SHEET TO A TEMPERATURE GREATER THAN 160* C., CONCURRENTLY DIRECTING A CURRENT OF AIR AT A TEMPERATURE LESS THAN 75*C. AT SAID SHEET AND ITS SUPPORTING GRID SURFACE TO MAINTAIN THE TEMPERATURE OF SAID SHEET BETWEEN 100 AND 160*C. AND TO MAINTAIN SAID SHEET IN CONTACT WITH SAID SUPPORTING GRID SURFACE. 